1. Field of the Invention
This invention relates to the production of radioactive isotopes and, more particularly, to a method for accelerator-driven system (ADS) isotope production by means of nuclear fission.
2. Background of the Invention
Radioactive nuclear isotopes are used in a variety of applications. One of the most important uses is in medicine wherein the isotopes serve either as tracers for diagnostic purposes or to attack tumor cells in which they are injected or otherwise implanted. Two isotopes that are frequently used are 99Mo (half life 66 hours) and 131I (half life 8 days). Given these short lifetimes, it is impractical to stock-pile them. They must be produced continuously and delivered quickly. Also, although the U.S. often imports these isotopes from abroad (Canada, Belgium), a sizable fraction of the radio activity of a shipment decays in transit. Currently most of the 99Mo used in the U.S. is supplied by the Chalk River reactor in Eastern Canada.
99Mo and 131I are not naturally occurring radionuclides nor are they the products of the radioactive decay of naturally occurring radionuclides. A way they can be produced is from the fission of fissionable isotopes such as 235U (239Pu can also be used). Irradiation or bombardment of fissionable material with neutrons, either in the core or in the reflector region of a nuclear reactor, is one method for inducing fission in specially designed production targets. However, most of the worldwide nuclear reactors used in the production of those isotopes are at, or even past, their design life expectancy. They are often shut down for rather long periods for repairs and maintenance. Many of these reactors are due for major refurbishment or decommissioning.
The 99Mo production targets used in reactors are mostly made of highly enriched uranium (90+ percent 235U). This is the same enriched uranium that is used in nuclear weapons; as such, its use poses a serious security threat. First, it is a target for rogue states or groups desiring to acquire nuclear weapons capability. Second, its introduction into an already critical reactor, without a careful analysis and safety review, increases the possibility of a runaway accident.
The introduction of isotope-producing 235U requires careful analysis and deployment for a safe operation. Moreover, isotope production is parasitical to other reactor uses. Finally, production of isotopes requires continuous operation of a nuclear reactor to meet demand.
The “ADONIS” project was pursued in Belgium as a method of producing 99Mo with an accelerator [See Y. Jongen, “A cyclotron driven neutron multiplier for the production of 99Mo,” at the 37th European Cyclotron Progress Meeting, Groningen, The Netherlands, Oct. 29, 2009].
Other accelerator-based methods of producing 99Mo are being considered. For example, electron beam accelerators can be used to drive the fission process in depleted uranium (238U). But this process requires a very large amount of beam power (˜75 MW) to produce 99Mo at the scale required in the U.S.
Another proposed approach is to irradiate a separated isotope 100Mo with high power electron beams (˜5 MW) to produce 99Mo via the photonuclear reaction 100Mo(γ,n)99Mo. It also requires the expensive separated isotope 100Mo as the target material to produce 99Mo as a very small component of the irradiated target of stable 100Mo. Extracting the essential daughter isotope 99mTc from such a low specific activity irradiated target is an undesirable feature of this process.
There is a need in the art for a method and a system to provide continuous and abundant production of Mo and other short-lived isotopes by means of an accelerator-driven system. The method should induce fission in fissile material in a manner that will not use weapons-grade material. The method should neither add to the production of nuclear waste, nor should it pose a danger of reaching criticality. The amount of fissile material used should be the minimum required to produce the required amount of isotope and the isotope produced should have a high specific activity as measured by the amount of isotope produced per gram of fissile material. The method should also be energy efficient as measured by the amount of isotope produced for a given electrical power used by the accelerator.